1. Field of the Invention
The present invention relates to a display device, and more specifically, relates to a display device in which a liquid crystal panel faces a touch panel, a protective panel, a front light unit, a back light unit and the like at a fixed gap.
2. Description of the Related Art
In a liquid crystal display device carrying out a line inversion drive, for example, a display device in which a touch panel was provided on the display side of a liquid crystal panel has had a problem that sound generation providing uncomfortable feeling was generated. The present inventors have marked the problem, have extensively studied it and as a result, considered that its cause is originated as below. Namely, it is considered that the surface (the surface of a polarized plate) of a liquid crystal panel and the rear face of a touch panel which faces the liquid crystal panel are charged by any reason and they are oscillated by electric field generated by the liquid crystal panel to cause the problem.
Its principle is specifically illustrated referring to the following drawings. For example, the arbitrary point a of the rear face of the touch panel 1 as shown in FIG. 14 and the point b of the surface of the liquid crystal panel 2 which faces the point a are marked, and when electric charge which is charged at the point a of the touch panel 1 is referred to as qa and electric charge which is charged at the point b of the panel 2 is referred to as qb, forces Fa and Fb which are respectively applied to these electric charges qa and qb are determined by the following formulae (further, components only to the lamination direction of the liquid crystal panel and the touch panel as the components of the forces (and the components of the electric fields) are considered).Fa=qa×Ea Fb=qb×Eb 
Further, Ea is electric field strength at the point a and Eb is the electric field strength at the point b.
Herein, the electric field strength Ea at the point a is determined by the following formula. (Further, the electric field strength has electric field components caused by signals applied to a pair of the electrode layers of the touch panel and the pixel electrode layers of the liquid crystal panel or segment electrodes, but since they are almost a case that they are not variations at a constant frequency which becomes the cause of the sound generation, they are daringly abbreviated hereat.)Ea=Ecoma(t)+Eqa+Eqb 
Furthermore, Ecoma(t) is caused by the drive of the liquid crystal panel (counter electrode layer) among the electric field components at the point a and is a function of time, Eqa is caused by the electric charge qa among the electric field components at the point a, and Eqb is caused by the electric charge qb among the electric field components at the point b.
Herein, Ecoma(t) is electric field which is generated by the counter electrode signals (hereinafter, referred to as CMOS signals) applied to the counter electrode layer. Since a voltage is changed at a constant frequency, the value of Ea is not constant and the CMOS signals are changed by drive frequency components. Accordingly, Fa is also changed by the fore-mentioned drive frequency. With respect to this, the force Fb which liberates the point b of the panel is also similar.
Further, considering the whole rear face of the touch panel, the total quantity of charge at the rear face of the touch panel is referred to as Qtp and the Qtp, namely Ftp which acts on the whole rear face of the touch panel is considered to be similarly changed. Similarly, when the total charge quantity of the whole surface of the panel is referred to as Q1c, F1c which acts on the whole surface of the panel is considered to be also changed.
Namely, when Ea and Eb are constant, the vibrations of the panel and the touch panel are not generated because the force acting on the Qtp and Q1c is constant, but since Ea and Eb are changed by the drive frequency which is a constant frequency as mentioned above, the surface of the panel or the rear face of the touch panel is vibrated by the same frequency as the drive frequency, it is considered that a space between the panel and the touch panel is vibrated to generate the sound generation.
The following methods are considered as a method of lowering and reducing such sound generation.
Firstly, there is a method of heightening or lowering the drive frequency of a liquid crystal panel which is a cause of the sound generation and excluding it from a range of audible frequency by a human or hardly hearing it. However, when the drive frequency is heightened, it causes the increase of power consumption and when drive frequency is raised in case of a liquid crystal display device, a charging time to a liquid crystal is shortened, therefore there occur problems that contrast is lowered because of the insufficiency of charging, cross stroke is generated and the like. Further, when the drive frequency is lowered, a problem of generating a flicker exists.
Furthermore, there is also considered a method of preventing the occurrence of the change of Coulomb's force at a panel face by making drive signals as DC drive. However, since the output voltage of a source is heightened in order to make the drive signals as the DC drive, a high voltage resistant article is required to be used as a source driver and there occurs a problem that power consumption is increased thereby.
Furthermore, there is also considered a method of removing an aerial layer between a liquid crystal panel and a touch panel by closely bonding the rear face of the touch panel on the surface of a liquid crystal panel with paste or the like and eliminating the vibration of the aerial layer to prevent the sound generation. However, when the touch panel and the like are directly bonded on the surface of the panel, there are the problem of display quality caused by peeling at an adhesive face and a problem that the bonding work is difficult.
Further, there is also considered a method of suppressing the charging at the surface of a liquid crystal panel and the rear face of a touch panel as low as possible and preventing the generation of vibration caused by Coulomb's force. However, it is required to carry out discharge working while carrying out the management of charging quantity by a discharge blower and the like in order to prevent the charging. There are problems of the lowering of productivity spent for the discharge working and the investment of a facility for managing the charging quantity.
Furthermore, there is also considered a method of suppressing the generation of vibration by separating a distance between a liquid crystal panel and a touch panel facing it and lowering the Coulomb's force, or a method of lowering the level of not hearing the sound generation caused by the vibration of air even if it is vibrated. However, there is a problem that a dimension to the lamination direction of a display device must be enlarged for thus separating the distance.
Further, a method disclosed in Japanese Patent Utility Model Publication (Unexamined) No. 7-20623 is known as a method of solving influence on the precision of positional detection by noise around a touch panel (a resistance membrane tablet device). It is disclosed in this patent literature 1 that a transparent electrode layer is formed on the surface of a touch panel and the transparent electrode layer is grounded. Further, since the charging is suppressed at the rear face of a touch panel (a face on which a transparent electrode layer was formed) in the method described in the fore-mentioned preceding literature, it is also considered that it is effective for the sound generation caused by the vibration of the rear face of a touch panel. However, when other member is additionally added on the rear face of the transparent electrode layer, it is considered that the sound generation as previously described is generated by the charging with such member.
The present invention was carried out considering the above-mentioned problems of a conventional technology, and the subject of the present invention is to prevent the sound generation generated in a display device equipped with a panel having a common electrode layer to which a voltage changing at a fixed frequency is applied like the liquid crystal panel and a tabular member which is arranged at a fixed gap from said panel.